It has been found expedient in the design of many state-of-the-art television receivers to utilize frequency synthesis techniques in the generation of the tuner's local oscillator signal. In particular, in a Phase-Locked Loop (PLL) synthesizer, a very stable, usually crystal-controlled reference signal is phase/frequency compared with the signal developed by a voltage-controlled oscillator (VCO). The voltage output of the comparator is appropriately filtered and coupled to the VCO so that its output frequency is locked and substantially equal to the operating frequency of the reference oscillator. In general the reference oscillator signal is divided by a fixed divider and the output of the VCO is divided by a variable divider, according to the desired frequency of operation, so as to supply the desired frequencies to the phase/frequency comparator. In a particular synthesizer system used in conjunction with the subject invention, a 4 MHz crystal-controlled reference oscillator is divided by a factor of 512 to produce a fixed reference frequency of 7.8125 KHz at one input of the phase/frequency comparator. The local oscillator frequency, at 45.75 MHz above the nominal picture carrier frequency for a given channel, is then divided by an appropriate factor to produce a signal of the same nominal frequency at the other input of the comparator. For example, the Channel 2 picture carrier is at 55.25 MHz and the receiver's local oscillator should be at 101 MHz. The appropriate divider factor is then 101 MHz/7.8125 KHz, or 12928. This factor can be implemented by a fixed divider at 64 followed by a variable divider set at 202. In a properly operating PLL synthesizer, any error in the receiver's local oscillator frequency will result in an output that will tend to equalize the frequencies of the signals at the input to the phase/frequency comparator, thereby correcting the local oscillator frequency.
It is clear that the system described above is satisfactory when the receiver is receiving signals whose picture carrier is at the frequency specified by the FCC for a given channel. In this situation the PLL operation will assure the desired local oscillator frequency and the picture and sound carriers will be substantially exactly converted to the standard IF frequencies, 45.75 MHz and 41.25 MHz respectively. However, when the television receiver is operating from sources, such as some CATV and MATV systems, that provide nonstandard carrier signal frequencies, the preprogrammed divider ratios described above will result in the conversion of the carrier signals to frequencies other than 45.75 and 41.25 MHz. In some cases the deviation from the specified standard frequencies may be as much as .+-.1 MHz. Those familiar with the design and operation of television receiver circuitry are aware that, because of the frequency selectivity of the receiver's signal processing circuitry, a significant deterioration of both picture and sound quality will likely result.
Prior art circuits that have indicated an awareness of and attempt to confront the above anomolies have not been entirely successful. For example, systems that switch from a crystal-controlled reference oscillator to some other, say, voltage-controllable reference when receiving a nonstandard signal, tend to lose tuning when the carrier signal is lost during the transition from crystal-controlled to voltage controlled reference oscillator operation. The momentary loss of tuning may be so significant so as to preclude subsequent reacquisition, thereby rendering the dual reference oscillator system fatally ineffective.